Field of the Invention
The present invention relates to separating a semiconductor layer from a substrate.
Description of the Prior Art
SiC substrates are commonly used for the high quality growth of semiconductor materials such as the homoepitaxial growth of SiC or heteroepitaxial growth of Group III-Nitrides (III-Ns) including gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and their various alloys for electronic or optoelectronic applications. Although SiC substrates offer advantages such as close lattice matching to the III-N material system for high quality epitaxial layer growth and excellent thermal conductivity for heat dissipation for power electronics, SiC substrates are relatively expensive. Therefore, it would be advantageous if a single SiC substrate could be reused for multiple semiconductor device layer growths. This could be accomplished by removing the grown semiconductor layers from the SiC substrate without damaging the SiC substrate or device layers and transferring them to a potentially less expensive substrate. Additionally, whereas SiC substrates may provide a suitable vehicle for semiconductor growth, separation of the semiconductor device layers from the SiC may be advantageous in certain applications, such as in III-N-based light emitting diodes (LEDs) where they typically destructively polish away SiC to extract light out of the bottom of the LED heterostructure.
Various methods have been reported to separate a semiconductor layer(s) from a substrate. A common separation technique is referred to as Smart Cut (U.S. Pat. No. 5,374,564). The Smart Cut process involves implanting ions (e.g. H+) into a donor substrate to create an abrupt, vertically localized, high concentration of ions at a targeted distance below the substrate surface forming a weakened layer. The surface of the donor substrate is usually capped with another material, such as a dielectric, prior to ion implantation to protect the surface. After ion implantation, the donor substrate is bonded to a carrier wafer and the two wafer system is heated to a temperature on the order of 400 to 600° C. The heating causes the implanted ions to coalesce and the original substrate to cleave parallel to the substrate surface along the weakened zone. Mechanical energy can be used instead of or in addition to thermal energy to split the substrate. After separation, the carrier wafer now possesses the thin top portion of the original substrate and that thin film can be further processed to fabricate the desired device. The original substrate can be recycled for subsequent Smart Cut processing. While the Smart Cut process is primarily associated with fabricating silicon-on-insulator wafers, this process has extended to various other materials such as SiC (U.S. Pat. No. 7,262,113) and GaN (U.S. Pat. No. 7,968,909).
There are various disadvantages of the Smart Cut method. After wafer splitting, the surface of both the remaining thin film on the carrier substrate and the donor substrate need to be polished to create a smooth, planar surface for subsequent processing, which adds additional processing steps. Part of the donor wafer is consumed for each Smart Cut process, limiting the lifetime of the donor wafer. The thickness of the transferred thin film is practically limited by the ion implantation equipment, typically to a few micrometers; this prevents the use of thicker films which may be desirable in certain applications. It may be advantageous to further process the thin film on the donor wafer prior to transferring it to the carrier wafer. Post ion implantation, processing the thin film on the donor wafer is limited by the thermal budget of the wafer splitting process, 400 to 600° C. This temperature range is below that needed for various semiconductor processing steps, such as ohmic contact annealing and dopant ion activation. The Smart Cut ion implantation step may cause damage to device layers if certain processing steps are done beforehand, such as gate oxide growth or deposition.
A method more specific to the separation of III-N materials from a substrate is the laser lift-off technique (U.S. Pat. No. 6,420,242). This process is used in the specific instance of a GaN layer grown on a sapphire substrate (other layers may be grown on top of the GaN, i.e. AlN). The sapphire substrate is irradiated with a laser at a wavelength that is transparent to the sapphire substrate but is absorbed by the GaN. The laser energy causes the GaN at the interface to decompose into Ga-rich regions. Further heating above the melting point of Ga (30° C.) causes the III-N material and the sapphire substrate to debond. The III-N material may be bonded to a carrier substrate prior to laser irradiation. This process has several limitations, particularly the selection of the substrate used for III-N material growth. Laser lift-off requires that the substrate have a larger band gap than the III-N layer grown on top. For instance while higher quality AlxGa1−xN films, where 0≦x≦1, can be grown on SiC compared to sapphire due to smaller lattice mismatch, the AlxGa1−xN films cannot be removed from the SiC substrate by laser lift-off due to the lower band gap of SiC. Most commercial GaN RF products use SiC substrates.
A sacrificial layer has also been used to separate III-N materials from a SiC substrate. In one process a smaller band gap material than the SiC substrate or other III-N epitaxial layers is grown first on the SiC as the sacrificial layer (U.S. Pat. No. 7,825,006). The sacrificial layer is removed using the aforementioned laser lift-off or photoelectrochemical (PEC) etching. In either case the design of the epilayer device structure is limited by the sacrificial layer, which must have the smallest band gap.
There are other known methods that combine ideas from the above. One is combining a sacrificial layer with the Smart Cut process to avoid consumption of the donor substrate (U.S. Patent Publication US2012/0309172). Another is ion implanting the substrate prior to growth to create a weakened zone, so that the grown layers and top of substrate fracture along the weakened zone after the growth process (U.S. Patent Publication US2006/0234486). These processes suffer similar limitations to those described above for the Smart Cut process.